Test Point Design for Probe and Fixture Access

Test points are the interface between the board and everything that has to prove the board works. They are usually added at the end of the layout, which is why they are often the reason a test fixture is difficult and a diagnosis takes longer than it should.

What the Test Point Is For

A test point gives a probe a defined place to contact. It is not the same as a via or a component pad, which may be masked, coated, or too small to contact reliably.

The point should be defined by its function: a node that needs to be measured in production, a node that needs to be measured during debug, or a node that only needs to be observed. The three have different requirements. Our flying probe notes describe how the two test methods differ.

Size and Shape

The pad must be large enough for the probe to land on it with the placement tolerance of the machine, and it must be at least as large as the probe tip plus the registration error.

A round pad gives a consistent contact; a square one gives a slightly larger target at the corner. Where the space is tight, a via without a mask tent can serve as a test point if the probe can reach it. Our fabrication notes notes list the attributes that should be stated.

Grid and Spacing

Probes are mounted on a fixture at a standard pitch, and the test points should follow that pitch wherever possible. Off grid points force a custom fixture and increase its cost.

The clearance between adjacent test points must exceed the probe body diameter, not the tip diameter. Two points that are close enough for the tips but not for the bodies cannot both be probed. Our design release notes cover where the requirement is recorded.

Access From One Side

A probe fixture contacts one side of the board at a time, so all the points that are tested together should be on the same side. A design that spreads them across both faces needs two fixtures or a double sided one.

Components on the probed side occupy the space the fixture needs. A tall part near a test point can be cleared by a cut-out, but the cut-out weakens the fixture and it should be designed in rather than discovered.

Masking and Coating

A test point must be left free of mask, and the mask opening must be larger than the pad so that the probe does not land on the mask wall. A probe that touches mask gives a reading that looks like an open circuit.

Where a coating is applied, the coating must be kept away from the test points or the points must be masked during application. Coating over a test point is the most common cause of an untestable board. Our coating inspection notes describe how the coverage is verified.

Electrical Considerations

A test point added to a high speed net adds capacitance and a stub. On a fast signal the stub changes the impedance and it can degrade the waveform more than the measurement is worth.

Where the net is sensitive, the test point should be placed on a short branch rather than on the main path, or the node should be tested through a buffer. Our high speed notes describe the routing measures that limit the effect.

Verification

The layout should be checked against the fixture requirement before the data is released, and the first fixture should confirm that every point can be contacted with the intended tolerance.

Where a point cannot be contacted, the fix is a layout change rather than a fixture modification, because the fixture will be rebuilt for every product revision. Our inspection notes describe the coverage that the optical system can add.

Process Control and Verification

On a design of this kind, grid is the item that decides how the rest of the board is arranged. Where the requirement is not written down, the shop supplies its own default, and the default is chosen for the process rather than for the design. The measurements that matter are the repeatable ones: conductor width and spacing, annular ring, finished hole size, plating thickness and surface finish are all verifiable on a coupon that travels with the panel.

Running a first article through the same checks as the production panel confirms that the two agree, and that comparison is the cheapest form of process control available at prototype stage. Keeping a sample from the panel turns a dispute into a measurement, because the same coupon can be re-examined by both parties without rebuilding the batch.

Reviewing the design before the data is released is cheaper than correcting it after the panel is in the tank, because every step downstream inherits the decision made at the front end. Documenting the assumption is part of the design work, and a short note on the drawing prevents a question that would otherwise arrive a day later and cost a day of schedule.

The process window is set by the narrowest step in the flow, so an improvement anywhere else shows up as margin rather than as yield until that step is addressed.

Process Control and Verification

On a design of this kind, grid is the item that decides how the rest of the board is arranged. Where the requirement is not written down, the shop supplies its own default, and the default is chosen for the process rather than for the design. The measurements that matter are the repeatable ones: conductor width and spacing, annular ring, finished hole size, plating thickness and surface finish are all verifiable on a coupon that travels with the panel.

Running a first article through the same checks as the production panel confirms that the two agree, and that comparison is the cheapest form of process control available at prototype stage. Keeping a sample from the panel turns a dispute into a measurement, because the same coupon can be re-examined by both parties without rebuilding the batch.

Process Control and Verification

On a design of this kind, grid is the item that decides how the rest of the board is arranged. Where the requirement is not written down, the shop supplies its own default, and the default is chosen for the process rather than for the design. The measurements that matter are the repeatable ones: conductor width and spacing, annular ring, finished hole size, plating thickness and surface finish are all verifiable on a coupon that travels with the panel.

A first article check confirms that the process and the drawing agree on the points listed above, and that the coupon data supports the values used in the design.

Where a measurement falls outside the expected window, the sample is retained so that the cause can be established before the balance of the batch is released.

FAQ

Can a component pad be used as a test point? It can where the pad is large enough and no component is fitted there. Once it is populated, it is no longer available.

Should test points be on the same grid as the probes? Yes wherever the layout allows, because off grid points are the main driver of fixture cost and of fixture lead time.

What does gopcb provide for test point design? We provide test point definition by function, pad size and shape for the probe tolerance, grid and spacing rules including probe body clearance, single side access planning, mask and coating clearance, high speed stub review, and fixture verification on the first build.